Those who have Turbofins have probably noted information from the instruction sheet pertaining to tuning the fins. It states "By having more tension in the fins, they become stiffer and faster with more resistance on the pedals." I decided to find out what might be the most efficient setting for cruising speeds.

Pedaling at 60 cycles per minute over a 107 yard course, I recorded boat speed for each setting: loose (see picture below), medium (tightened 1/8") and tight (tightened an additional 1/16"). Each setting was run four times on each fin independently to minimize error.

Resulting speeds varied only about 2%; pedal effort was noticeably different though. The tighter the fin clew, the more effort was needed to maintain pedaling speed. This effect was more pronounced with the front fin than the rear fin. Technically, the fastest speed obtained was, front fin = loose; back fin = medium. This makes sense, as the rear fin gets a deflected slipstream from the front fin. The effect was weak though and may not be worth the extra pedal effort.

The bottom line is, loose clews give virtually the same performance for significantly less effort at cruise speeds. Less effort translates to less fatigue over longer distances.

What about sprint speeds? Sorry to say, I actually went faster with relaxed clews (loose fins) than tight fins. Like a propeller blade, the fins need to be able to develop a pitch (i.e. flex) to generate forward thrust. Very tight clews restrict pitch and end up wasting energy by batting water back and forth.

What is the optimal setting? I like to set the back edge of the fin about 1/8 inch in the outhaul slot, just enough so it doesn't pop out when flexed. See picture above.

As a second check, I look for at least 1/4 inch fin retraction space on the mast (if possible) so it can "wind up" the mast to allow the necessary flex as it moves from side to side. This is also a good way to set up the fins on the older style Drive (different outhaul).

Speed limitations? Even with speeds exceeding 9 MPH on boat wakes and 11 MPH on the AI (sailing), the fins are still pulling when set this way.

Are the Outhaul Adjustment Screws a waste? Not at all! There have been small changes to the mast lengths and fin styles. Additionally, there is some suggestion that the rear fin may benefit slightly by a little tighter setting. The Adjustment screws allow corrections for all of the above, as well as accommodating personal tastes of different individuals and further experimentation.

Many months ago I took the 2 Adventures with turbo fins to Rainbow River. We had to pedal upstream against a 1-2mph current and after 1/2 hour the turbo fins were wearing out our legs. Removed drives and saw that the clew adjusters were very tight. Loosened them up and we both noticed a remarkable difference. Easier to pedal but without any noticable loss in speed.

No speed runs or time trials done by me; I was pretty well convinced by that river episode.

Thanks for your testing, pics and confirmation about the clew adjusters.

I bought some longer stainless screws (with nylock nuts) for the clew, to allow more rotation of the fin. I turned the middle of the screws down slightly (hourglass shape) and put a thin rubber ring on each side (of the adjuster screw eye) to keep it quiet. You can shape the screw down (just enough to get rid of the threads in the middle of the slot) on a grinder or even with a hand file.... it only takes a little to free up the motion.

This allows even lower pitch of the fins; much more in keeping with our relatively slow boat speed. Just look at a good sailboat prop, and the little light bulb will go on over your head (nobody else can actually see that little bulb, you know).

This little trick has been the BEST improvement I've found (after the Turbo Fins). Try it yourself, only costs a couple bucks.

So - tried adjusting my ST trurbo fins as suggested. I agree that it seems to require less effort at low to moderate speeds (up to about 4.5 mph).

However, it seems to me that it also requires a slightly higher cadence, especially as speeds get higher. I haven't yet tried anything scientific, since we had brisk winds and a rough chop on top of good sized waves and my bow kept getting submerged. Tough to get meaningful numbers.

Has anyone else seen this effect?

I think what we really need is an adjustment that tightens up the clew as speed increases - essentially a variable pitch prop. I'll add that to my list of design features for my titanium / carbon fiber mirage drive replica

So - tried adjusting my ST turbo fins as suggested. I agree that it seems to require less effort at low to moderate speeds (up to about 4.5 mph).

However, it seems to me that it also requires a slightly higher cadence, especially as speeds get higher.

Yes, that's the case -- the faster the cadence, the faster the speed (given the same fins and stroke length).

Quote:

I think what we really need is an adjustment that tightens up the clew as speed increases - essentially a variable pitch prop.

I think we already have a variable pitch prop -- the harder you push, the more the twist, the bigger the bite. But unlike a conventional prop, you can't really over-twist it since the twist ratio is not rigidly built in. The question is, how much pitch should you start out with for best efficiency? IMO, more is better; loose clews put the root of the fin to work and add potential pitch to the entire fin.

The following mod is somewhat similar to JimL's. This can increase cruise speeds by about .1 to .2 MPH and still retain full power at sprint speeds.

Take a pair of lock rings and attach them to the clew adjustment screws as shown below. This clears the fins of the clew outhauls and allows them to articulate and swing more from side to side. This works with screws, but is a little smoother if you're able to use clevis pins:

Then, slip the rings over the clevis pins (or screws if you're using them). This is what it looks like on the newer drives. Notice that the outhaul is free to pivot out more:

And here it is on an older Drive:

By adjusting the clew screw it can be set up for varying degrees of freedom. IMO, this is one of the rarely tapped potentials of the Mirage Drive. The only disadvantage I've noticed is that you can hear the fins clicking from side to side. JimL's rubber bumpers should fix that though!

Last edited by Roadrunner on Thu Jul 31, 2008 10:31 pm, edited 1 time in total.

I guess I wasn't clear about my cadence question. I did figure out early on that faster cadence means faster speed

What I was trying to say is that with more slack in the clew adjustment, it requires a higher cadence to go the same speed than it does with less slack. This difference seems more noticeable to me at higher speeds. Just wondered if anyone else had the same observation.

I also have the variable pitch concept, I think. The harder you push, the more the clew deflects, and the steeper the pitch - sort of like using a lower gear. This is exactly what you want when accelerating from a standstill, for instance. I'm wondering if the ideal setting for higher speed cruise might be a bit tighter, though.

I'm still a little on the outs on the the Turbo fin thing it seems like some have many problem with them.

I feeling like they(Turbo ST fins) may be putting a bit too much of a load on the drive system the ST's work good and can be adjusted.

Are the Turbo fins really worth the appearent teething problems experienced by some I know some have said they will never go yakking without they're turbo fins so lets hear some more thoughts on this subject.

I tried to address this in another thread with a list of reasons NOT to get the ST Turbo fins. Reason number 2:

2) If you have really strong leg muscles and testosterone poisoning (or are likely to loan your 'yak to someone who does). The Turbo offers enough resistance to place a good deal more strain on the drive.

I think Hobie has made changes that strengthen up the weak spots, but if you want to prove how strong you are I think the Turbo fins give you a better chance of breaking the drive. If your interest is 100 yard sprint races, I'd worry a bit. For normal use they seem to be fine.

One of the biggest beefs I have with the Stainless Steel Sprockets is the mast wobble even if the cotter pin is new. The wobbling mast justs gets worse over time enlarging the hole in the SS sprockets even more. This HAS to be a real loss of effeciency.

Yes, I prefer the SS over the plastic sprockets. My heavy turbo fin use, coupled with the ability to strike objects, re-straighten the mast and keep going are better than taking a chance of cracking a plastic sprocket.

I am very happy Hobie decided to keep making the Stainless Sprockets to keep us power users happy.

Now, how about just welding the turbo mast to the SS sprocket?!

Don't bother cutting notches or drilling holes in the turbo masts and don't bother drilling hole in SS sprocket for a cotter pin - both saving on machine shop work. Weld the turbo mast to SS Sprocket - making it one unit.

No cotter pin to break! No more mast wobble!
Solid, Stable, Strong!

Bent masts will just get straightened in a vise.

You can even make a non-turbo version combo, but the real beneifit is for the turbos.

So how about it Hobie? Can we PLEASE get a SS sprocket & turbo mast as 1 piece?

Are the Turbo fins really worth the appearent teething problems experienced by some I know some have said they will never go yakking without they're turbo fins so lets hear some more thoughts on this subject.

If performance is important to you, get the Turbos. If you don't want anything to happen to your drive, lube, inspect and maintain it regularly, just like you would any mechanical device. If the Drive is going to have a problem, 9 out of 10 times it will give an advance warning. Even then, 9 out of 10 failures the Drive is still operable and can safely get you home. Your chance of having a problem with Turbos, especially with the "08 drum upgrade is very small.

IMO, the only current potential issue with Turbos is the possibility of a mast wallowing and eventually slipping out where it inserts into the sprocket. Not only does it give ample warning, but is easy to address with a dab of epoxy in the shaft.

STs aren't a bad alternative, and standard fins might be a best choice where the environment is going to beat up the fins (oyster bars, etc.). But for speed, range and acceleration, Turbos are the way to go. I've been using them for over 2 years now (since they first came out) and absolutely love them!

In two years of Turbo Fin use I've only had one problem after slamming into an unseen submerged object, at full power in ,San Francisco Bay last spring.
It would have broken the mast on any other size fin as well so its not really a Turbo fin specific incident.
However $15.00 in parts and my labor fixed the sprocket and mast and there have since been no other problems to report .
I also used a blue thread locker on the set screws that hold the masts in place and have no issues with hole hogging of the nylon mast sprocket to report as well.
Our Turbo fins have simply been trouble free and our mirage drives as well have had no serious issues .
Kepnutz

Thanks Roadrunner for explaining the tuning of the turbo's. After following your advice I realised I had mine up way too tight and loosened them up. Made pedaling much more comfortable!
Looks like Hobie's new sprocket/mast reinforcement is another step towards bulletproofing the drive. Thanks Hobie for the ongoing improvements!